At present, high-energy elements, for example iodine-131 element, are more and more widely used in nuclear medicine, and especially application of iodine-131 to thyroid diseases, such as diagnostic iodine-131 systemic imaging, post-treatment iodine-131 systemic imaging, removal of residual thyroid tissue after thyroid cancer surgery and treatment of recurrence and metastatic tissue with iodine-131, etc., has a significant effect in treatment with safety and convenience, and is thus a radionuclide therapy project developing most rapidly in China.
A patient treated with iodine-131 needs to be monitored for radiation dose residues for better observation and analysis of the disease condition. At present, an distribution image of iodine-131 in a human body is obtained by γ-ray imaging technology in nuclear medicine. Once iodine-131 nuclide decays in the human body, γ particles are released in random directions. After the γ particles directly facing a detector pass through the human body, they will be captured by a crystal and release a certain number of photons. A photoelectric sensor converts the photons into electrons and outputs the same in the form of current pulses to a preamplifier circuit. The current signal is converted by an integration circuit into a voltage signal. Here, the capability of the γ particles is linearly correlated with the number of the photons and the amplitude of the voltage signal. Through the above process, a counter (an arithmometer) is added on a corresponding position of the image picture. With an increase of the duration of the acquisition, the count at different positions of the image increase constantly. The magnitude of the counted values at each position is linearly correlated with the amount of iodine-131 at a corresponding position of the human body. In this manner, the counted values are converted to a grayscale image, which is the distribution image of iodine-131 in the human body.
Specifically, the γ particles released from iodine-131 mainly contain particles of two energy levels: 284 keV and 365 keV, while the proportions of particles of other two branches of energy levels (80 keV and 723 keV) are very small (negligible in an actual acquisition process). In an actual detection process, after the γ particles of the same energy are acquired, a normally distributed sharp peak is formed. The ratio of the width at half peak value to the peak value is the so-called energy resolution in nuclear physics. In general, the narrower is the energy peak value, the smaller is the energy resolution value, and the better is the energy resolution performance.
Iodine-131 releases γ particles in random directions, and the released γ particles will be subjected to Compton scattering after colliding with an object. In an energy spectrogram of the γ particles released from iodine-131, the left side of the full energy peak is a Compton plateau that is generated from the Compton scattering effect. The γ-rays generated from the scattered γ particles are not what we need and need to be filtered in the subsequent circuit part by a window setting. Otherwise the finally generated iodine-131 distribution image would be blurred, and the spatial resolution of the image would be affected. In order to improve the acquisition efficiency and thus acquire all the γ-rays of various energy levels, it is necessary to adequately increase the width of the window in the circuit. Moreover, considering that the number of γ-rays at a low energy level of 284 KeV is small and their contribution in the energy spectrum falls within the Compton plateau, it is thus difficult to distinguish the energy peak of the γ-rays at 284 KeV from the Compton plateau. Thus, by increasing the width of the window, the number of the scattered ineffective γ-rays which are marked as effective γ-rays also increased therealong, which ultimately affects the spatial resolution of the image. In view of the above, the prior residue scanning device still has the problems of low detection efficiency and poor spatial resolution.